Celtic mythology is the mythology of Celtic polytheism, the religion of the Iron Age Celts. Like other Iron Age Europeans, the early Celts maintained a polytheistic mythology and religious structure. Among Celts in close contact with Ancient Rome, such as the Gauls and Celtiberians, their mythology did not survive the Roman Empire, their subsequent conversion to Christianity, and the loss of their Celtic languages. It is mostly through contemporary Roman and Christian sources that their mythology has been preserved. The Celtic peoples who maintained either their political or linguistic identities (such as the Gaels in Ireland, and the Brittonic tribes of Great Britain) left vestigial remnants of their ancestral mythologies, put into written form during the Middle Ages.